IEEE ANTENNA AND WIRELESS PROPAGATION LETTERS 2013
1
Unveiling Magnetic Dipole Radiation in
Phase-Reversal Leaky-Wave Antennas
arXiv:1401.0242v1 [physics.class-ph] 1 Jan 2014
Shulabh Gupta, Member, IEEE, Li Jun Jiang, Senior Member, IEEE, and Christophe Caloz, Fellow, IEEE
Abstract— The radiation principle of travelling-wave type
phase-reversal antennas is explained in details, unveiling the
presence of magnetic-dipole radiation in addition to well-known
electric dipole radiation. It is point out that such magneticdipole radiation is specific to the case of traveling-wave phasereversal antennas whereas only electric-dipole radiation exists in
resonant-type phase-reversal antennas. It is shown that a phasereversal travelling-wave antenna alternately operates as an array
of magnetic dipoles and an array of electric-dipoles during a
time-harmonic period. This radiation mechanism is confirmed
through both full-wave and experimental results.
scanning. This configuration uses periodic phase-reversal radiating elements interconnected by balanced transmission line
sections [10].
c1
mutually cancelling
magnetic dipoles
conventional differential lines
λ/2
+
−
c2
+
−
=
λ/2
+
Index Terms— Balanced transmission line, full-space scanning,
leaky-wave antenna, phase-reversal, magneto-electric antennas.
−m
m
−
(a)
phase-reversal antenna at t = 0
I. I NTRODUCTION
Periodic leaky-wave structures are a versatile class of
travelling-wave antennas capable of frequency-controlled
beam steering, offering high directivity and simple feeding
mechanism [1]. With the advent of composite right/left-handed
(CRLH) leaky-wave antennas capable of full-space scanning,
from backward to forward including broadside directions,
there has been a renewed interest in such structures [2].
Full-scpae scanning was made possible by suppressing the
open stopband and equalizing the impedance around broadside [2][3][4]. Following these advances, periodic leaky-wave
antennas have become most attractive as low-cost alternative
to phased array antennas [5].
One approach to achieve a full-space beam scanning in
leaky-wave structures, is to use the phase-reversal technique.
This technique involves introducing a 180◦ phase shift along
the axis of the structure, so that λg /2-spaced periodically
elements get excited in phase and therefore radiate. This technique is common in slot-waveguide antenna arrays where the
required extra 180◦ phase-shift is induced by proper location
of the slots along the waveguide aperture [6]. A transmissionline counterpart of this structure is the Sterba-curtain antenna,
which utilizes periodic transmission-line cross-overs, placed
λg /2 apart, to produce electric-dipole radiation from the crossovers [7]. However, these waveguide and transmission-line
antennas were resonant in nature and therefore featured a
small impedance-matching bandwidth product [8],[9]. This
phase reversal concept was later extended to a travellingwave configurations, thereby both improving the impedancematching bandwidth performance and providing full-space
S. Gupta and L. J. Jiang are with the Electrical and Electronic Engineering
Department of University of Hong Kong, China (SAR).
C. Caloz is with the Department of Electrical Engineering, PolyGrames
Research Center, École Polytechnique de Montréal, Montréal, QC, Canada.
radiating magnetic dipoles
+
−
+
−
=
m
m
+
−
(b)
radiating electric dipole
phase-reversal antenna at t = T /4
+
−
−
+
−
=
+
+
p
−
p
(c)
Fig. 1. Principle of phase-reversal antenna explained in terms of current
distributions in two adjacent unit cells. a) Differential transmission line
without phase reversal. b) Phase reversed differential lines at t = 0 and, c) at
t = T /4, where T = 1/f0 is the time period of an assumed time-harmonic
signal of frequency f0 .
While the radiation mechanism of traditional transmissionline type phase-reversal antennas is well understood in resonant configurations and extensively discussed in the literature
[11], travelling-wave phase-reversal antennas have not been
exhaustively explored. Specifically, they have been described
in terms of electric-dipole radiation only [10]. In this work,
the radiation mechanism of a travelling-wave type phase
reversal antenna is revisited in details and unveils the presence
of magnetic-dipole radiation operating in conjunction with
electric-dipole radiation. This dual radiation mechanism is
specific to travelling-wave type phase-reversal antennas and
IEEE ANTENNA AND WIRELESS PROPAGATION LETTERS 2013
2
x
h
p = λg /2 at f0
x
s
y
z
w
g
y
∆ℓ
(a)
x
x
x
z
z
−30°
0°
30°
−60°
60°
−90°
0°
30°
−60°
−30°
60°
90° −90°
−20
120°
−150°
0dB
−90°
120°
−150°
0dB
−150°
30°
−60°
60°
90°
0dB
f = f0
120°
−10
150°
M-dipole Gθ (θ)
φ = 90◦ (y − z plane)
f < f0
0°
120° −120°
−10
150°
Gy (f > f0 )
−20
−120°
E-dipole Gφ (φ)
θ = 90◦ (x − y plane)
M-dipole Gθ (φ)
θ = 90◦ (x − y plane)
−30°
z
90° −90°
−10
150°
30°
−20
−120°
−10
0°
60°
−20
−120°
z
Gz (f > f0 )
−60°
90°
x
z
Gy (f = f0 )
Gz (f = f0 )
−30°
x
z
Gy (f < f0 )
Gz (f < f0 )
x
−150°
0dB
150°
E-dipole Gφ (θ)
φ = 0◦ (x − z plane)
f > f0
(b)
Fig. 2. Planar transmission line implementation of a phase-reversal leaky-wave antenna and its typical radiation pattern characteristics [10]. a) Antenna
layout. b) 3D polar plot and the corresponding 2D radiation patterns in three principal cuts, computed using FEM-HFSS.
does not exist in their resonant counterpart. They fall in
the category of magneto-electric dipole antennas, where the
electric- and magnetic-dipoles radiate together from a single
radiating structure [12], [13]. The detailed principle of the
phase-reversal antennas is explained next.
II. P RINCIPLE
OF A
P HASE - REVERSAL L EAKY- WAVE
A NTENNA
Consider a matched transmission line consisting of two
closely-spaced conductors of length ℓ = λ at a frequency f0 ,
as shown in Fig. 1(a). This transmission line can be considered
as two half-wavelength long lines connected in tandem via an
ideal transmission line of zero length. Such a line supports a
differential mode, since the current distributions on the top and
bottom conductors are mirror images of each other. In such a
configuration, the pair of parallel half-wavelength transmission
lines may be seen as one-wavelength long current loops, c1 and
c2 , forming two adjacent magnetic dipoles, m, as illustrated
in Fig. 1(a). These two magnetic dipoles are anti-parallel, and
therefore do not radiate since they produce mutually cancelling
far fields.
Fig. 1(b) shows a modified configuration the differential
line, where a π-phase shift, commonly called phase-reversal, is
introduced by crossing the connections between the conductor
pairs. Consider a time-harmonic signal, of period T . At a
certain time instant t = 0, the two adjacent magnetic dipoles
become parallel to each other, as a result of phase reversal
which produces magnetic-dipole far-field radiation. At that
instant, the currents in the cross-over region are minimal and
out-of-phase, and therefore do not radiate. The phase-reversal
antenna may thus be seen as a series-fed array of magneticdipole antennas at time t = 0.
At a later time instant t = T /4, the current distribution on
the phase-reversal antenna changes as shown in Fig. 1(c). In
this case, the current currents are maximal and in-phase at
the cross-over region, leading to the formation of a radiating
electric dipole moment, p, perpendicular to the axis of the
lines. While part of the current distribution on the transmission
lines may be mapped to small current loops, the corresponding
adjacent magnetic dipole moments are out-of-phase and therefore do not radiate. The phase-reversal antenna may thus be
seen as a series-fed array of electric-dipole antennas, at time
time t = T /4 [10].
IEEE ANTENNA AND WIRELESS PROPAGATION LETTERS 2013
x
3
z
50Ω to 100Ω microstrip to differential transition
y
100Ω termination
ℓ = 34 cm (20 cross-overs)
(a)
measured Gθ (θ, φ)
measured Gφ (θ, φ)
−10
−15
−20
−25
−30
4.5
4.75
5
5.25
0.2
−10
0.4
−20
0.6
−30
0.8
5.5
0
0.5
0.2
−10
0.4
−20
0.6
−30
0.8
0
1
Azimuth angle φ × π rads
frequency f (GHz)
(b)
0.5
1
Azimuth angle φ × π rads
(c)
M-dipole scanning
φ = 90◦ (y − z plane)
E-dipole scanning
φ = 0◦ (x − z plane)
E- and M-dipole
θ = 90◦ (x − y plane)
90°
90°
60°
0
0
Elevation angle θ × π rads
Elevation angle θ × π rads
Measured
FEM-HFSS
−5
S11 (dB)
0
0
0
90°
60°
120°
(dB)
120°
60°
0
120°
-10
30°
30°
150°
150°
30°
150°
-20
-30
±180°
0°
±180°
0°
± 180°
0°
-40
-30
−30°
−30°
−150°
−60°
−120°
−90°
Gφ (dB)
−150°
−60°
Measured
FEM-HFSS
−120°
−90°
−30°
Gθ (dB)
−150°
−60°
Measured
FEM-HFSS
−120°
-20
-10
0
−90°
(d)
Fig. 3. Fabricated phase reversal antenna and measured results. a) Photograph of the antenna. b) Measured S-parameters. c) Measured antenna gain and d)
2D radiation patterns in all three principal planes, for the transition frequency f0 = 4.8 GHz. The layout parameters are: p = 16.51 mm, g = 0.8 mm,
∆ℓ = 1.27 mm, w = 1.27 mm, s = 0.254 mm and number of cross-overs N = 20.
III. R ADIATION C HARACTERISTICS
AND
R ESULTS
Based on the above explanation, it is clear that a phasereversal antenna is based on two distinct radiation mechanisms. It alternates between magnetic-dipole radiation and
orthogonally-polarized electric radiation, with a combination
of the two in between these two instants. To further investigate the radiation properties of a phase-reversal leaky-wave
antennas, designed in planar transmission line technology, is
shown in Fig. 2(a). The antenna consists of two conducting
strips on each side of a dielectric substrate, with a strip width
w and offset g, to form a transmission line of characteristic
impedance Z0 . The cross-over region is realized using a small
transmission line section of length ∆ℓ with an impedance Zc
such that Zc 6= Z0 . This cross-over region with an impedance
step acts as a matching element to suppress the well-known
open stopband, typical of periodic structures [1], as proposed
and demonstrated in [10]. Under the condition of complete
stop band suppression, there is a zero phase shift across the
unit cell at the design frequency, f0 , which is sometimes
also referred to as the transition frequency of the antenna,
corresponding to peak radiation at broadside.
Typical radiation patterns for the phase-reversal leaky-wave
antenna at the transition frequency are shown in Fig. 2(b),
shown also in a 3D polar plot for better visualization. The
electric and magnetic dipole radiation components are clearly
apparent in the x − y plane. While a y−directed electric
dipole, from the cross-overs, produces a maximum along the
x−axis, the x−directed and orthogonally polarized magnetic
dipole produces a null in this direction. Similarly a magnetic
dipole maximum along the y-axis aligns with the minimum
IEEE ANTENNA AND WIRELESS PROPAGATION LETTERS 2013
of the y−directed electric dipole. In the other two planes, the
magnetic-dipole component of the phase-reversal antenna with
a maximum along the y−direction forms a directive beam in
the y − z plane, and its orthogonally-polarized electric-dipole
component with a maximum along the x−direction forms a
directive beam in the x − z plane. These two planes are also
the frequency scanning planes where the magnetic-dipole array
and the electric-dipole array scans in the y − z and the x − z
planes, respectively as seen in Fig. 2(b).
To further confirm the radiation characteristics of the phasereversal leaky-wave antenna, a prototype is built on an FR4
substrate (εr = 4.4 and h = 0.8 mm), as shown in Fig. 3(a).
It is fed through a multi-stage quarter-wavelength transformer
to excite the differential mode of the phase-reversal antenna
using a 50 Ω microstrip feed [10]. Its measured return loss
S11 is shown in Fig. 3(b) showing a satisfactory matching,
with S11 < −10 dB within the band of interest. Fig. 3(c)
shows the measured radiation map of the antenna gain at
f0 = 4.8 GHz, for its two orthogonal components Gφ and Gθ .
The corresponding radiation patterns in three principal plane
cuts are plotted in Fig. 3(d). As expected, two distinct sets of
radiation patterns, corresponding to the magnetic-dipole array
and an electric-dipole arrays are clearly seen in all planes with
peak gains of 1.2 dB and 2.3 dB, respectively. A reasonable
agreement is also observed between the measured and the fullwave simulated results, with a stronger agreement at higher
gain values, due to increased measurement sensitivity.
IV. C ONCLUSIONS
The radiation principle of a travelling-wave type phase
reversal antenna has been explained in details, unveiling the
presence of magnetic-dipole radiation which exist in addition
to the electric-dipole radiation. This is in contrast to the
resonant configuration where only electric-dipole radiation
exists. It has been shown that the radiation characteristics of
a phase-reversal antenna alternates between that of an array
of magnetic dipoles and of an array of electric dipoles. While
the crossover regions provide the electric-dipole radiation, the
balanced transmission line between the cross-overs form a
wavelength-long current loop at the designed frequency, representing the magnetic dipole radiation. This radiation principle
has been confirmed using both full-wave and experimental
results. This insight into the phase-reversal type antenna
highlights the rich radiation characteristics of phase-reversal
antennas compared to what was previously understood, and
may lead to efficient and multifunctional leaky-wave antennas.
ACKNOWLEDGEMENT
Authors would like to thank the Radio-frequency Radiation
Research Laboratory at the Chinese University of Hong Kong
for their help in radiation pattern measurements. This work
was supported by HK ITP/026/11LP, HK GRF 711511, HK
GRF 713011, HK GRF 712612, and NSFC 61271158.
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